1. Field of the invention
The present invention relates to optical waveguides, and more particularly, to plastic optical waveguides of which the refractive index can be freely controlled and which have improved resistance to heat.
2. Prior art
Together with the implementation of optical communication systems made possible through the development of low loss optical fiber, there has been an accompanying need for the development of various components employed in such optical communication systems. Particularly, there has been considerable demand for optical circuits and components therefor which support high throughput of communication signals, especially so for optical waveguides applicable to such systems.
In general, properties considered to be requisite for optical waveguides include: 1) low optical losses; 2) facile fabrication; 3) controllable core-cladding refractive index ratio; and 4) high heat resistance.
Efforts to develop optical waveguides with low optical losses have chiefly centered on implementations incorporating quartz glass. Due to the excellent permeability to light of quartz glass, it has been possible to achieve low optical losses of 0.1 dB/cm and less at a wavelength of 1.3 .mu.m in optical waveguides fabricated therefrom. Manufacture of quartz glass optical waveguides, however, presents a number of problems including a lengthy manufacturing process in terms of time, the need for high temperatures during fabrication, and the problem of increased length which is necessary with quartz glass optical waveguides. In response to these problems, attempts have been made to produce optical waveguides using plastics such as polymethylmethacrylate (PMMA), which can be carried out at low temperatures. With conventional plastic optical waveguides, however, resistance to high temperatures is poor and optical loss in near infrared region is large.
Among the various organic polymers currently available, polyimide provides very good resistance to heat, for which reason these materials has been widely employed in fields of avionics, satellite technology and so on. In recent years particularly, it has been expected that polyimide materials would offer a variety of useful properties above and beyond resistance to heat, and come to be employed in an even wider range of applications. For example, it has been hoped that polyimides having a low coefficient of thermal expansion and low dielectric constant could be developed for use in circuit boards, LSI chips and the like as an interlayer dielectrics therefor. Polyimides having a low refractive index would be useful in optical communication applications, particularly so as cladding for optical waveguides. Additionally, there is a need for polyimides with a low water absorption.
Unfortunately, polyimides which adequately fulfill the above described needs have yet to be developed. It is believed that a polyimide of which the main chain is sufficiently rigid should provide a material having a suitably low coefficient of thermal expansion, and that monomers such as tetracarboxylic dianhydrides or diamines should have substituents which provide a suitably low dielectric constant and refractive index. For example, for epoxy resins, as reported in the Journal of Polymer Science, Part C, and in Polymer Letters, Vol. 24, No. 249 (1986), by using a polyfluorinated substituent for the curing agent, epoxy resins having a lowest dielectric constant. Further, as disclosed in Japanese Patent Application, First Publication No. Sho-61-44969, epoxy resins having a lowest refractive index using a polyfluorinated substituted base.
In consideration of the above discussion, it can be seen that shortcomings exist with conventional optical waveguides formed from quartz glass or plastic, and that these conventional optical waveguides do not adequately meet the previously described four properties considered essential for commercially produced optical waveguides, namely, low optical losses, facile fabrication, controllable core-cladding refractive index ratio and high heat resistance.
In addition, there have been no reports describing to control the refractive indices using not less than 2 kinds of polyimides having different fluorine contents one another.
In Japanese Patent Application, No. Hei-1-201170, the present inventors have disclosed that fluorinated polyimide prepared using 2,2'-bis-(trifluoromethyl)-4,4'-diaminobiphenyl have an extremely low dielectric constant and refractive index, and furthermore, have excellent transparency characteristics. Additionally, some of these highly transparent fluorinated polyimides can be dissolved in suitable solvents.
One possible application for which the disclosed fluorinated polyimides could be employed is the manufacture of optical waveguides. For the manufacture of optical waveguides, however, it is necessary to be able to freely control the core-cladding refractive index ratio, and when using the disclosed fluorinated polyimides, the refractive index can be coarsely varied over the range of from 1.49 to 1.71. Additionally, by virtue of having core and cladding layers with different refractive indices, a multilayer structure is necessary with such fluorinated polyimide optical waveguides. Because the disclosed highly transparent fluorinated polyimides are soluble in solvents of polyimide precursor, however, forming such a multilayer structure is difficult. As a result, optical waveguides formed from transparent polyimides which are soluble in organic solvents have not as yet been achieved.